The posters above are making good points all valid and have recommended you go to a walk on . Forget the field ,for a moment and possible injuries and fallout. If 20 of you want to play and frankly less would make all the hassle and preparation not worthwhile .You will need 20+markers .I say+ since most site level markers will not last a day without at least1 of 20 going down . 20+masks ,20+ air tanks .A small compressor and dive tanks or lots of dive tanks ,with the small fill adaptor .....starting to see the other posters point?? 20 boxes of paint for the day .If you 're not involved in the sport ,you wont know any outlets who will sell you a box for around 20/25 a box .So you'll buy mail order prices or some ghastly field crap. You will have to organise ,reff, fetch n carry the lot .If it rains and you're off in the distant field ,you will get soaked .I could go on ,but if you have markers and masks ,for about 50£ at most walk on sites you can have fun games ,marshalled ,tech advice if your gun breaks ,food on site paint box included .You will also get to meet other ballers (possibly the biggest free bonus of the day) and if as is common in balling half of the "Im definitely coming mates " dont show ,you can join in and play with these people . Do yourselves a favour and find out paintball has a lot to offer beyond just your first punter day .Try a walk on .